Process for preparing novel high solids non-aqueous polymer compositions

ABSTRACT

The use of falling-film evaporation and a concentration means such as centrifugal separation to dehydrate and concentrate a water-in-oil polymer emulsion or suspension is disclosed. In a preferred embodiment, a water-in-oil polymer composition containing less than about 25% active polymer solids is dehydrated in falling-film evaporator to produce an evaporator concentrate containing less than about 5.0% water. The evaporator concentrate then flows to a separation device such as a centrifuge, wherein oil is removed to produce a composition that is substantially a polymer and emulsifying surfactant-in-oil composition containing in excess of about 60 weight % polymer solids. The centrifuge raffinate may optionally be treated in a second separation means to capture additional polymer solids, with optional recycle of the oil-rich raffinate. The concentrated polymer composition is preferably stabilized with a steric stabilizer to produce a free flowing liquid product.

This is a continuation of application Ser. No. 07/993,990 filed Dec. 12,1992, now abandoned.

BACKGROUND OF THE INVENTION

Water soluble polymers can be prepared in several forms. For instance,it is known in the art that water-soluble polymers can be dispersed intothe oil phase of water-in-oil emulsions. Conventional emulsionpolymerization techniques generally involve either:

(1) forming a water-in-oil emulsion of an aqueous solution of at leastone water-soluble ethylenically unsaturated monomer in an inerthydrophobic liquid organic dispersion medium and polymerizing saidmonomer or monomers in said dispersion medium to form a polymeremulsion; or

(2) forming a water soluble polymer and then emulsifying the polymersolids into an emulsion having a hydrophobic liquid as the continuousphase.

In such polymerization methods, virtually any water soluble monomercontaining a CH₂ ═C functional group can be polymerized and dispersedinto a water-in-oil system. Thus, anionic, cationic, nonionic andamphoteric emulsion polymers can be prepared by conventional emulsionpolymerization techniques.

Water-in-oil emulsions are generally comprised of three primarycomponents including: (1) a hydrophobic phase; (2) an aqueous phase; and(3) a surfactant system. The hydrophobic phase generally comprises fromabout 5 to about 75%, by weight, of the emulsion. Any inert hydrophobicliquid can be used as the hydrophobic phase. Preferred hydrophobicliquids, for example, are selected from the group consisting of benzene,xylene, toluene, mineral oils, kerosene, napthas, petroleums, and blendsof aromatic and aliphatic hydrocarbons containing 4 or greater carbonatoms. Particularly preferred hydrophobic liquids are those selectedfrom the group consisting of a narrow fraction of a branch-chainhydrocarbon sold by Witco Chemical Company under the tradename Kensol61®and branch-chain isoparafinic hydrocarbons sold by Exxon under thetradename Isopar®.

The aqueous phase generally comprises from about 25 to about 95%, byweight, of the emulsion. This phase comprises the desired ethylenicallyunsaturated monomer or monomers and water. Additionally, this phase maycontain an effective amount of a chelating agent, such as a sodium saltof ethylene diamine tetracetic acid (EDTA) or nitrilotriacetate (NTA),buffers, chain transfer agents or other additives. The monomer(s) to bepolymerized generally comprise about 10 to about 60%, by weight, of theaqueous phase, with the balance being primarily water.

The surfactant system generally comprises about 0.5 to about 20%, byweight, of the emulsion. Any surfactant system which effectivelydisperses an aqueous phase into a hydrophobic phase can be used. See,for example, U.S. Pat. No. 3,826,771, which discloses the use ofsorbitan monooleate as water-in-oil emulsifying agent; U.S. Pat. No.3,278,506, which discloses the use of ethylene oxide condensates offatty acid amides as emulsifiers; U.S. Pat. No. 3,284,393, whichdiscloses the use of hexadecyl sodium pthalate, sorbitan monooleate,sorbitan monostearate, cetyl or stearyl sodium pthalate and metal soapsas water-in-oil emulsifiers; and U.S. Pat. No. 4,024,097, whichdiscloses the use of surfactant systems comprising an oil-solublealkanolamide, such as Witcamide® 511, which is commercially availablefrom Witco Chemical Company, and one or more co-emulsifiers selectedfrom the group consisting of unesterified dialkanol fatty amides,quaternized ammonium salts of fatty tertiary amines, salts of fattytertiary amines, alkaline metal salts of fatty acids and alkyl oralkylaryl sulfates or sulfonates.

U.S. Pat. Nos. 4,672,090 and 4,772,659 disclose a surfactant systemwhich comprises 25 to 85%, by weight, an oil-soluble alkanolamide; 5 to35%, by weight, a polyoxyethylene derivative of a sorbitan ester; and 0to 50%, by weight, sorbitan monooleate. Additionally, other surfactants,such as sorbitan monostearate, may be used in combination with theseprimary surfactants.

Emulsion polymers are generally produced by first mixing the surfactantsystem with the hydrophobic phase. The aqueous phase is then typicallyprepared by adding the monomer(s) to water in the desired concentration.Additionally, a chelant, such as a sodium salt of EDTA, may be added tothe aqueous solution and the pH of the aqueous phase may be adjusted tobetween about 3.0 and 10.0, depending on the monomer(s) used. Theaqueous monomer phase is then added to the mix containing thehydrophobic liquid and the surfactant system. The surfactant systemenables the aqueous phase, which contains the monomer or monomers to bepolymerized, to be emulsified into the hydrophobic phase. Polymerizationis then carried out in the presence of a free radical generatingcatalyst, and the temperature of the reaction mixture is generallymaintained between about 5° and about 100° C., preferably between about20° and about 50° C., resulting in water-in-oil emulsion polymer. Anytype of free radical initiator can be used, including, for examplepersulfate and azo initiators. High energy irradiation can also be usedto initiate polymerization.

The following patents provide further background relative to themanufacture of emulsion polymers.

U.S. Pat. No. 3,041,318 discloses emulsifying an aqueous dispersion of apolymer prepared from a compound containing a CH₂ ═C group into awater-in-oil emulsion, and then precipitating the polymer therefrom asdiscrete particles.

U.S. Pat. No. 3,284,393 discloses a water-in-oil emulsion polymerizationprocess wherein a water-soluble monomer is emulsified in an oil phaseand polymerized therein, resulting in a polymeric latex which is thenseparated from the reaction medium.

U.S. Pat. Nos. 3,624,019 (reissued and reexamined as B1 Re 28,474) and3,734,873 (reissued and reexamine as B1 Re 28,576) disclose thepreparation of water-in-oil emulsions of vinyl addition polymers usingvarious surfactants, particularly low HLB surfactants, as emulsifiers.The polymers are dispersed into emulsion by means of agitation. Thus,polymers which are powders or "lumplike agglomerates" are comminuted anddispersed into the oil phase using mechanical energy. The specificationat column 3 states that "The invention contemplates using emulsionscontaining between 5-75 percent by weight with preferred emulsion havinga polymer concentration within the range of 10-45% by weight. In somecases the starting emulsions are converted to suspensions due to thenature and the amount of the polymer present therein." Though purelytheoretical polymer concentrations of up to 75% are recited, theactivity range found in the examples is 23-37%.

U.S. Pat. No. 3,826,771 discloses "high solids" water-in-oil emulsionswhich have a polymer concentration between about 20 and 50%, by weight,based on emulsion weight. This reference discloses that the method ofthe U.S. Pat. No. 3,041,318 cannot be used to prepare emulsionscontaining 20 to 50%, by weight, of a vinyl addition polymer having amolecular weight in excess of 1,000,000 because aqueous solution cannotbe prepared containing high enough concentrations of the polymer.

U.S. Pat. No. 3,888,945 discloses the use of azeotropic distillation toremove water from a suspension of an aqueous solution of a polymer in anon-polar suspending medium.

U.S. Pat. No. 3,997,492 discloses stable water-in-oil emulsion polymerswhich have polymer concentration between 10 and 50%, by weight, of theemulsion.

U.S. Pat. No. 4,021,399 discloses a method for concentrating awater-in-oil emulsion polymer by distilling under vacuum whilemaintaining the water-to-oil phase ratio substantially equal to that ofthe starting latex. Though this method is stated to be useful inpreparing emulsions containing up to 70%, by weight, polymer, theexamples are limited to ≦48% active polymer.

U.S. Pat. No. 4,035,317 discloses a method for preparing free-flowingsolid polymer particles from water-in-oil emulsions. A hot gas stream isused for drying.

U.S. Pat. No. 4,052,353 discloses the treatment of water-in-oilemulsions via evaporation to reduce the water content thereof and toproduce a polymer stable to coagulation or agglomeration. The use offalling-film evaporation is not disclosed or suggested.

U.S. Pat. No. 4,035,347 discloses a method for preparing substantiallydry polymers from water-in-oil emulsions using a thin film dryingtechnique.

U.S. Pat. No. 4,528,321 discloses the distillation of dispersions toremove water and other volatile components. Theoretical polymerconcentrations as high as 85%, by weight, are disclosed. However, noexamples are given showing greater than 57% polymer.

UK Pat. No. 1,482,515 discloses water-in-oil emulsions whichtheoretically contain up to 63% active polymer.

U.S. Pat. No. 4,506,062 discloses a reverse phase suspensionpolymerization process and the use of azeotropic distillation to preparepolymer dispersions in oil which theoretically contain up to 70% polymersolids. In various examples the polymer dispersions of the earlierexamples are centrifuged to assess particle size range and shape. Acentrifuge is used in a standard compaction test; this reference doesnot, however, disclose the use of a centrifuge to concentrate anemulsion. The process is believed to degrade molecular weight andsolubility properties.

U.S. Pat. No. 5,155,156 discloses a process wherein the water in apolymer latex containing water and an emulsifier is evaporated, followedby separation of the dried polymer from the emulsifier via filtration orcentrifugation, multiple washing steps and redispersion of the polymerinto a second oil. The water content of the polymer after drying is"most preferably about 3% to 8%" and this reference does not disclosethe use of falling-film evaporation to dehydrate polymer emulsions.Though 85% active polymer compositions are claimed, the maximum solidsconcentration obtained in the examples is 68%.

In summary, the inventors believe that the references discussed aboveprovide a comprehensive overview of the emulsion polymerization art, butthat they, alone or in combination, do not disclose or remotely suggestthe instant invention. While certain of these references suggest, inpurely theoretical terms, emulsions containing in excess of 70% activepolymer, stable emulsions at such concentrations are simply notcommercially available. This is verified by the examples of the abovereferences, which only show the preparation of emulsion polymerscontaining less than about 70% active polymer.

Commercial emulsion polymer compositions contain substantial portions ofwater and are much less active. For example, U.S. Pat. No. 4,035,317states that: "Two problems still existed, however, in the first place,the cost of transporting the emulsion is materially higher than if thepolymer per se were being transported. Since the emulsions contain, onthe average, only about 25-30% polymer, 70-75% of the material shippedis useless oil and water. Secondly, the emulsions can present astability problem when exposed to temperatures below their freezingpoint."

Water soluble polymers containing in excess of about 60% solids arecurrently only commercially available in dry or gel form. Dry polymersare generally prepared by drying conventional solution polymers, andgels are simply high viscosity polymer solutions which can not betransported as liquids. They are generally adherent, tough, rubberysubstances which retain their shape when placed on flat surfaces. Gelsare not generally used in gel form but instead as dilute solutions ofthe polymer. Dilution methods oftentimes involve mechanical steps whichresult in the degradation of certain properties of the polymers, such asmolecular weight.

To obtain polymers having high molecular weights and good watersolubility properties, it is necessary to prevent crosslinking reactionsduring polymerization to the extent possible. In this respect, it isdesirable to prepare polymers under relatively mild conditions, e.g.,conducting polymerization reactions using relatively low monomerconcentrations.

Unfortunately, aqueous solution polymerization techniques which use lowmonomer concentrations require that the resulting solution polymers bedried to prepare powdered polymers, thus resulting in increasedmanufacturing costs and degradation of polymer properties such asmolecular weight and solubility.

Though there is a long felt need in industry for stable, high molecularweight, soluble, easily handled polymer delivery systems containing inexcess of 70% active polymer, this need has not been met because of theart's inability to efficiently concentrate liquid polymer compositionsto a stable form having a high polymer content without substantiallydegrading the resulting high solids polymers.

Also, while dry polymers provide high activity, handling and make-downproblems remain. A major benefit of the instant polymer compositions isthat the economic advantages of dry polymers compared to emulsion andsolution polymers can be realized along with the additional advantagesassociated with emulsion polymerization techniques, such as that watersoluble polymers of high molecular weight are prepared in a pourable,stable liquid form. This invention advances the art of polymerpreparation and delivery in that stable, pourable, high molecularweight, water soluble polymer compositions containing in excess of about60%, preferably in excess of 70%, active polymer are now available.

It is also noteworthy that no prior art reference known to the inventorsdiscloses or suggests: 1) the use of falling-film evaporation todehydrate emulsion polymers, particularly to water contents of less than3%, based on total composition weight, and 2) the use of a separationmeans such as a centrifuge to further concentrate a dehydratedpolymer/emulsifying surfactant/hydrophobic liquid composition producedvia a falling-film evaporator into a high solids end use product. Thisfurther supports the novelty of the instant invention, which relates tothe use of a falling-film evaporation and additional steps to dehydrateand concentrate starting water-in-oil emulsions, dispersions orsuspensions, thereby producing non-aqueous polymer/emulsifyingsurfactant/hydrophobic liquid compositions containing less than about 5%water, preferably less than about 3% water, and greater than 60%,preferably greater than 70%, polymer solids.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a flow diagram illustrating the instant process forpreparing non-aqueous, high solids, stable, pourable polymeremulsions/suspensions.

SUMMARY OF THE INVENTION

The instant invention relates, in general terms, to non-aqueouspolymer/emulsifying surfactant/ hydrophobic liquid compositions and tothe preparation of such compositions via the use of falling-filmevaporation and an additional separation step such as centrifugation,filtration or evaporation to dehydrate and concentrate water-in-oilpolymer compositions. The resulting polymer compositions, which containless than 5%, preferably less than 3%, more preferably less than 2%,water, and greater than 60%, preferably greater than 70% polymer solids,based on total composition weight, are pourable, stable, high solidscationic, anionic, nonionic or amphoteric polymer emulsions orsuspensions which are substantially non-aqueous.

Conventional commercially available emulsions or suspensions of a watersoluble polymer in a continuous oil phase generally only contain up toabout 30-50 weight % polymer solids. This means that 50-70 weight % ormore of such emulsions is inactive. Nonetheless, the convenience of suchcompositions mandates their wide use in industry.

The instant inventors have discovered that falling-film evaporators,preferably when used in conjunction with one or more separation stepsfor the removal of hydrophobic liquid, such as filtration, evaporationor centrifuge steps, are particularly well suited to remove water fromwater-in-oil polymer compositions. Because falling-film evaporatorsefficiently remove water from such compositions without thermallypunishing the polymer contained therein, falling-film evaporationenables the preparation of stable, high solids polymer compositionswhich are substantially non-aqueous and which generally retain themolecular weight and solubility advantages of the starting emulsion.

Ideally, in the instant dehydration/concentration process, one startswith a water-in-oil polymer suspension, dispersion or emulsion having apolymer concentration of less than about 25%, preferably about 1 toabout 15%, based on total suspension or emulsion weight. If the polymerconcentration of the composition to be dehydrated and concentratedexceeds about 25%, evaporator fouling tends to become troublesome. Insuch cases, dilution, preferably using a suitable hydrophobic liquid, toa polymer concentration of less than about 25% is recommended. Thewater-in-oil polymer composition containing less than about 25% polymersolids is then dehydrated in a falling-film evaporator, wherein thewater content of the composition is lowered to about 5% or less,preferably 3% or less, more preferably to between about 0.1 and about2.5%, most preferably to between about 0.1 and about 1.5%, based ontotal composition weight, followed by concentration via one or moreseparation steps to remove hydrophobic liquid, such as centrifuge,evaporation and/or filtration steps. Centrifugal separation ispreferred. Also, a steric stabilizer is preferably added to reduce thepotential for compaction and/or separation of the concentratedcomposition.

Falling-film evaporators are used for the dehydration step of theinstant process because they are believed to minimize heat punishment ofthe polymer composition being dehydrated. Since polymers are heatsensitive, this preserves the molecular weight and solubility propertiesof the starting emulsion. In a falling-film evaporator, the startingemulsion is exposed to high temperatures, preferably but not necessarilyunder vacuum, for relatively short periods of time. This enablesfalling-film evaporators to quickly flash water off of starting polymercompositions without thermally degrading the polymer or substantiallyeffecting the stability of the starting emulsion.

After dehydration, the evaporator concentrate is treated via a secondseparation process, such as a filtration, evaporation or centrifugationprocess, to remove additional hydrophobic liquid. Preferably, theevaportor concentrate is centrifuged to produce what is substantially apolymer and emulsifier-in-oil composition containing in excess of about60% polymer solids, preferably in excess of about 70% polymer solids,and less than about 5%, preferably less than about 3%, and morepreferably less than about 2%, water. The raffinate from the centrifugecan be optionally fed to a second separation device such as a centrifugeto capture additional polymer solids, with the oil-rich raffinate streamof the second centrifuge being recycled. Finally, the polymer andemulsifying surfactant-in-oil composition produced by the centrifugestep(s) can be stabilized against compaction or separation using astabilizer such as a steric stabilizer.

The instant process is designed to substantially increase the polymerconcentration of standard commercially available emulsions, suspensionsor dispersions without negatively impacting the performance of thepolymer. This means that, on an active basis, the volume of materialwhich must be handled is significantly reduced, which in turn makes theinstant polymer compositions valuable in applications where standardemulsions are not suitable because of excessive storage requirements,such as off-shore oil drilling platform applications. In addition, theinstant compositions compete effectively with dry polymers due to theirhigher molecular weights, improved performance, and comparable storagespace requirements. The instant products are also believed to offeradvantages over standard emulsions relative to convenience, stability,compaction, oil separation and reduced susceptibility to gel formationduring storage because of the low moisture content of the material.

DETAILED DESCRIPTION OF THE INVENTION

In its broadest sense, the instant invention relates to non-aqueous,high solids polymer compositions containing less than about 5%,preferably less than about 3%, by weight, water and greater than 60%,preferably greater than about 70%, polymer solids, and to thepreparation of the same through the use of falling-film evaporation andone or more separation steps, preferably centrifuge steps, to dehydrateand concentrate a starting water-in-oil polymer emulsion or suspension.

The resulting compositions, which may be emulsions, suspensions ordispersions, are stable, pourable compositions which are essentiallynon-aqueous and which contain in excess of 60% polymer solids and lessthan about 5% water, preferably in excess of 70% polymer and less thanabout 3% water. More preferably, these compositions contain less than 2%water.

Further, the instant invention relates to a process for removing waterand increasing the polymer concentration of a water-in-oil polymercomposition comprising a water soluble polymer, one or more emulsifyingsurfactants, water and a hydrophobic liquid, which method comprises: a)dehydrating said composition via an evaporation step to form apolymer/emulsifying surfactant/hydrophobic liquid composition, and b)centrifuging, evaporating or filtering said polymer/emulsifyingsurfactant/ hydrophobic liquid composition to remove some portion ofsaid hydrophobic liquid therefrom, thereby producing a concentratedpolymer/emulsifying surfactant/ hydrophobic liquid compositioncontaining less than about 5%, preferably less than about 3% and morepreferably less than about 2%, based on total composition weight, ofwater. These concentrated polymer/emulsifying surfactant/hydrophobicliquid compositions generally contain in excess of about 60% polymersolids, preferably in excess of about 70% polymer solids. Preferably, asstep c), an effective amount of a steric stabilizer is added to theconcentrated polymer composition to improve its stabilitycharacteristics.

The instant invention is also directed to a water solublepolymer/hydrophobic liquid/emulsifying surfactant composition orsuspension which contains less than about 5%, preferably less than about3%, water and greater than about 60% polymer solids, based on totalcomposition weight. More preferably, these compositions contain lessthan about 2% water and greater than about 70% polymer, based on totalcomposition weight.

Additionally, the instant invention is directed to a process forconcentrating a starting water-in-oil polymer composition comprising: 1)about 25 to about 95%, based on total composition weight, of an aqueousphase comprising about 10 to about 60%, based on the weight of 1), of awater soluble polymer, and the balance water; 2) about 0.5 to about 20%,based on total composition weight, of at least one emulsifyingsurfactant; and 3) about 25 to about 95%, based on total compositionweight, of a hydrophobic liquid, which method comprises: a) dehydratingsaid composition using an evaporator, preferably a falling-filmevaporator, to produce a polymer/emulsifying surfactant/hydrophobicliquid composition which is substantially water free, i.e., whichcontains less than about 5%, preferably less than 3%, more preferablyfrom about 0.1 to about 2.5% water, and most preferably from about 0.1to about 1.5%, water; b) centrifuging said polymer/emulsifyingsurfactant/hydrophobic liquid composition so as to remove some portionof said hydrophobic liquid therefrom, thereby producing a concentratedpolymer/emulsifying surfactant/hydrophobic liquid composition and acentrifuge raffinate, wherein said concentrated polymer/emulsifyingsurfactant/hydrophobic liquid composition contains less than about 5%,preferably less than about 3%, more preferably less than about 2%,water, greater than about 60%, more preferably greater than about 70%,polymer and about 0.1 to about 10% emulsifying surfactant; c) optionallytreating said centrifuge raffinate in a second separation device, suchas a centrifuge, filter or evaporator, preferably a centrifuge, therebyproducing a second concentrated polymer composition and a secondraffinate stream; d) optionally blending said second concentratedpolymer composition from said second separation device with saidconcentrated polymer/emulsifying surfactant/hydrophobic liquidcomposition and recycling or disposing of said second raffinate stream;and e) optionally adding an effective amount of a steric stabilizer tothe resulting concentrated composition.

In this process, the evaporator is preferably a falling-film evaporator,more preferably a thin film evaporator, the centrifuge of b) ispreferably a decanter centrifuge and the centrifuge of c) is preferablya partially ejecting disk centrifuge. Also, if the polymer concentrationof the starting water-in-oil polymer compositions exceeds about 25%,based on total composition weight, the starting composition ispreferably diluted with a suitable hydrophobic liquid oil so as toreduce the polymer concentration to less than about 25%, preferably tobetween about 1 to about 15% by weight, based on total compositionweight. Compositions produced by this process are also claimed.

The instant invention is further directed to a method for removing waterfrom a water-in-oil polymer composition which comprises passing saidcomposition through a falling-film evaporator. Thus, this inventioninvolves the use of a falling-film evaporator to dehydrate a startingwater-in-oil polymer emulsion, dispersion or suspension and to thedehydrated product produced thereby. Preferably the startingwater-in-oil polymer emulsion, dispersion or suspension contains lessthan 25%, based on the total weight of the starting emulsion, dispersionor suspension, polymer. In cases where the starting polymerconcentration is higher, a hydrophobic oil dilution step is preferred.Thin film evaporators are the most preferred type of falling-filmevaporator. The products produced by falling-film evaporators preferablycontain less than about 3% water, more preferably between about 0.1 andabout 2.5% water, and most preferably between about 0.1 and about 1.5%water.

The instant invention also relates to a method for concentrating adehydrated polymer composition by removing hydrophobic liquid therefrom,which comprises treating a dehydrated polymer composition (e.g., anemulsion, dispersion or suspension), particularly the dehydratedcomposition produced by a falling-film evaporator, in at least oneconcentration device such as a centrifuge, evaporator or filter,preferably a centrifuge and more preferably a decanter centrifuge, toproduce a concentrated polymer/emulsifying surfactant/hydrophobic liquidcomposition. The resulting compositions also comprise an integral partof this invention. Preferably, the resulting composition contains inexcess of about 60%, more preferably in excess of about 70%, polymersolids and less than about 5%, more preferably less than about 3% andmost preferably less than about 2%, water. Decanter centrifuges havebeen found by the inventors to be especially well suited to concentratedehydrated emulsions.

Still further, the instant invention relates to the use of a secondseparation device such as a filter, evaporator or centrifuge, preferablya centrifuge, to concentrate the raffinate of a first centrifuge used toconcentrate a polymer emulsion or suspension, which allows foradditional polymer capture and oil recycle. A partially ejecting diskcentrifuge is preferred for this application.

The inventors believe that any type of water-in-oil polymer compositioncan be concentrated by the instant process. Thus, as used herein, theterm "water-in-oil polymer composition" includes virtually anywater-in-oil emulsion, dispersion or suspension containing a watersoluble polymer prepared from an ethylenically unsaturated monomer ormonomers, wherein said polymer is a portion of an aqueous phase which isdispersed, emulsified or suspended by one or more surfactants in asystem having a hydrophobic liquid as the continuous phase, and whereinthe polymer comprises less than about 60%, by weight, of thecomposition.

The water-in-oil polymer compositions discussed herein can contain anytype of water soluble polymer prepared from one or more ethylenicallyunsaturated monomers. Thus, the polymers may be nonionic, cationic,anionic or amphoteric. The instant invention is generally applicable,for example, to nonionic and anionic water soluble emulsions orsuspensions of ethylenically unsaturated polymers prepared from, interalia, N-vinyl pyrrolidone, N-vinyl formamide, ethoxylated acrylate andmethacrylate esters such as hydroxyethyl methacrylate (HEM) and the 5,10 and 20 mole ethoxylates of HEM, acrylamide, methacrylamide,N,N-dimethylacrylamide, N-i-propylacrylamide, N-tert-butylacrylamide,acrylic acid, α-halo acrylic acid, maleic acid or anhydride, itaconicacid, vinyl acetic acid, allyl acetic acid, methacrylic acid,acrylonitrile, vinyl sulfonic acid, allyl sulfonic acid, vinylphosphonic acid, vinyl acetate, 2-acrylamido-2-methylpropane sulfonicacid (AMPSA), 2-methacrylamido-2-methylpropane sulfonic acid(methAMPSA), styrene sulfonic acid, hydroxyalkyl acrylates,β-carboxyethylacrylic acid, β-sulfoalkyl (meth)acrylates, sulfoalkylmethacrylates, allyloxy-2-hydroxypropane sulfonic acid, andmethacrylamido hydroxypropyl sulfonic acid, alone or in combination, andsalts of such polymers.

The polymers of this invention may also be cationic, such as polymersmade from dimethyldiallyl ammonium chloride (DMDAAC), diethyldiallylammonium chloride (DEDAC), dimethyldiallyl ammonium bromide (DMDAAB),diethyldiallyl ammonium bromide (DEDAAB),methyacryloyl-oxyethyltrimethyl ammonium chloride (METAC),acryloy-oxyethyltrimethyl ammonium chloride (AETAC),methacryloy-oxyethyltrimethyl ammonium methosulfate (METAMS),acryloyoxyethyltrimethyl ammonium methosulfate (AETAMS),methacrylamido-propyltrimethyl ammonium chloride (MAPTAC),dimethylaminoethylacrylate, dimethyl aminoethyl methacrylate,dimethylamino propylmethacrylamide, alone or in combination, andpolymers made by polymerizing the above listed cationic monomers withnonionic monomers such as any of the above listed nonionic monomers,particularly acrylamide, methacrylamide or N,N-dimethylacrylamide.

Any amphoteric polymer can be used in the instant compositions,including, but are not limited to, polymers made by polymerizing one ormore of the above listed anionic monomers with one or more of the abovelisted cationic monomers, and especially polymers made from acrylicacid, methacrylic acid and/or AMPSA or methAMPSA and DMDAAC, AETAC,METAC, METAMS, and/or MAPTAC. Optionally, nonionic monomers may beinclude Also, betaine polymers can be made fromN(3-sulfopropyl)-N-methacryloyloxyethyl-N,N-dimethyl ammonium betaine,alone or in combination with various nonionic monomers.

Generally, the anionic, nonionic, cationic or amphoteric polymerspresent in the instant water-in-oil polymer compositions have molecularweights greater than about 100,000, as determined by viscometry.Preferably, molecular weights range from about 500,000 to about50,000,000.

The instant process involves the concentration of a starting polymeremulsion or suspension to greater than about 60% polymer solids,preferably to greater than about 70% polymer solids. The resultingpolymer compositions are in a stable, pourable form, and the instantprocess generally does not substantially alter the properties of thepolymers in the starting emulsion, particularly the molecular weight orsolubility characteristics of these polymers.

The compositions produced by the instant process are novelpolymer/emulsifying surfactant/hydrophobic liquid compositions whichcontain less than about 5%, preferably less than about 3%, and morepreferably less than about 2%, based on total composition weight, water.These compositions also comprise: a) at least about 60%, by weight, awater soluble polymer, preferably at least about 70%, by weight,polymer; b) about 10 to about 35%, by weight, hydrophobic liquid; c) atleast about 0.1%, by weight, of at least one emulsifying surfactant; andoptionally, d) an effective amount of a steric stabilizer, allpercentages being based on total composition weight.

More preferably, the instant non aqueous compositions also comprise: a)about 70-85% water soluble polymer; b) about 10 to about 25% hydrophobicliquid; c) about 0.1 to about 10% of at least one emulsifying surfactantstabilizing polymer; and d) optionally, an effective amount of a stericstabilizer. Generally, inert materials are also present.

To prepare the instant compositions, a "starting composition" which maybe a polymer emulsion, dispersion or suspension, comprising an initialweight concentration of water, an initial weight concentratior of ahydrophobic liquid, an initial weight concentration of at least oneemulsifying surfactant, and an initial weight concentration of a watersoluble polymer is dehydrated and concentrated by a process whichcomprises: a) diluting the starting emulsion, if the initial polymerweight concentration exceeds about 25% of said water-in-oil polymercomposition, with a suitable hydrophobic liquid so as to produce awater-in-oil composition having a polymer weight concentration of fromabout 0.1 to about 25%, preferably from about 1 to about 15%, based oncomposition weight; b) dehydrating the composition of step a) byevaporating water from the water-in-oil polymer composition of step a),preferably via falling-film evaporation, thereby lowering said initialweight concentration of water to less than about 5%, preferably lessthan about 3%, more preferably to between about 0.1 and about 2.5% andmost preferably to between about 0.1 to about 1.5%, and producing anevaporator concentrate; and c) treating the evaporator concentrate ofstep b) to remove some portion of the hydrophobic liquid from saidevaporator concentrate, for example by filtration, evaporation orcentrifugation, preferably by centrifugation, thereby producing aconcentrated composition which is substantially a water soluble polymerand emulsifying surfactant suspension, dispersion or emulsion in ahydrophobic liquid, and a raffinate, preferably a centrifuge raffinate.The raffinate can optionally be treated in a second separation devicesuch as a filter, centrifuge or evaporator, preferably a centrifuge, tocapture additional polymers solids and to reclaim additional hydrophobicliquid.

The water-in-oil polymer composition containing a water soluble polymerprepared from an ethylenically unsaturated water soluble monomer ormonomers to be concentrated generally should not contain an invertingsurfactant, since inverting surfactants hinder effective dehydration inthe evaporating step, particularly via falling-film evaporation. Asidefrom this limitation, it is believed that virtually any water-in-oilpolymer composition can be concentrated. For example, commerciallyavailable water-in-oil emulsion polymer suspensions or emulsions can beused. As examples of commercially available water-in-oil polymercompositions, one may cite the Pol-E-Z® and HYDRAID® emulsion polymersoffered by Calgon Corporation.

As indicated above, such emulsions or suspensions comprise a continuoushydrophobic liquid phase, at least one emulsifying surfactant and anaqueous phase containing water and a water soluble polymer. In the eventthat the starting water-in-oil polymer composition has an initialpolymer weight concentration of greater than about 25%, based on thetotal weight of said water-in-oil polymer composition, the startingemulsion or suspension is preferably diluted with a suitable hydrophobicliquid so as to reduce the initial polymer weight concentration to belowabout 25%, i.e. to from about 0.1% to about 25%, preferably from about 1to about 15%, based on total composition weight. As used herein, theterm "suitable hydrophobic liquid" refers to any hydrophobic liquidwhich is compatible with the water-in-oil polymer composition to beconcentrated. The same hydrophobic liquid that is used in the startingemulsion or suspension is preferred. If the starting water-in-oilpolymer composition contains less than about 25% active polymer solids,the dilution step is generally unnecessary.

Using either a starting water-in-oil polymer composition having aninitial polymer weight concentration of less than about 25%, or using awater-in-oil polymer composition which has been diluted with a suitablehydrophobic liquid so as to produce a water-in-oil polymer compositionof less than about 25 weight % polymer, the instant process preferablyrequires dehydration via the use of a falling-film evaporator so as toremove water from the initial water-in-oil polymer composition having apolymer weight concentration of less than about 25%, thereby loweringthe initial weight concentration of water to less than about 5%,preferably to less than about 3%, more preferably to between about 0.1to about 2.5% and most preferably to between about 0.1 to about 1.5%,and producing an evaporator concentrate.

Falling-film evaporators expose the emulsion, dispersion or suspensionto be concentrated to high temperatures, preferably under vacuum, whichcauses water to be quickly flashed off. Because residence times aretypically short, the polymers so treated are not thermally punished toany substantial extent. The evaporator concentrate produced generallyexits the evaporator at greater than about 200° F., which means thatcooling prior to subsequent processing is preferred. Conventional heatexchange techniques can be used to cool the evaporator concentrate,preferably to less than about 150° F.

The preferred type of falling-film evaporator is a thin film evaporator.Thin film evaporators have a unique ability to handle heat-sensitive andtime-at-temperature sensitive substances. In a typical thin filmevaporator, the starting polymer emulsion or suspension enters the unit,which preferably but not necessarily operates under vacuum, above thethermal section and is distributed via the action of fixed clearancerotor blades. As the emulsion flows down the walls of the evaporator, arolling fillet or bow front is generally formed on the leading edge ofthe blades. The turbulence and mixing action imparted to the fluid isbelieved to result in high heat and mass transfer rates withoutlocalized overheating. The dehydrated fluid exits the bottom of theunit, with a small fraction of vaporized oil, entrained emulsion and thewater vapor exiting the top.

After the preferred cooling step, the evaporator concentrate preferablypasses into a centrifuge. Though, in theory, any type of centrifuge canbe used to further concentrate the evaporator concentrate, adecanter-type centrifuge is preferred for this operation.

A decanter centrifuge is characterized by a cylindrical-conical rotatingbowl. The high rotational speed of the bowl develops a centrifugal forcewhich causes the polymer solids in a composition fed to the unit to bethrown out against the bowl wall, forming an annular ring. A helicalscrew conveyor fits inside the bowl with a small clearance between itsouter edge and the inner wall of the bowl. The conveyor rotates ateither a slightly lower or higher rate than the bowl. This difference inspeed, known as the differential speed, causes the separated solids tobe conveyed toward the conical end of the machine where they aredischarged. The lighter liquid phase overflows through ports at theopposite end of the machine.

The centrifuge should have the ability to produce a centrifugal forceequivalent to from between about 500 and about 10,000 times gravity,preferably from about 1,000 to about 6,000 times gravity. Thecentrifugal action causes the larger, heavier polymer solids to bescrolled out of the centrifuge, thereby producing a concentrated polymerand emulsifying surfactant-in-oil composition which contains in excessof about 60% polymer solids, preferably in excess of about 70% solids,and virtually no water. This primary effluent from the centrifuge is aviscous, pourable polymer/emulsifying surfactant/hydrophobic liquidcomposition having a viscosity of approximately 300 cps to 25,000 cps.These compositions generally contain less than 3% water, preferably lessthan 2% water, based on total composition weight.

As such, these compositions are referred to as non-aqueous, high solidspolymer compositions. As used herein, the term "non-aqueous" refers topolymer compositions which contain less than 5% water. Additionally, acentrifuge raffinate is produced which may be further concentrated via aconcentration means such as a filter, evaporator or another centrifuge,preferably by a second centrifuge. Again, it is believed that any typeof centrifuge can be used to concentrate the raffinate from the firstcentrifuge. However, a disk type centrifuge, preferably a partiallyejecting disk centrifuge, has been found to be preferred for this secondcentrifugal separation. The first centrifuge raffinate generallycontains less than about 5 weight % polymer solids. This stream can beconcentrated via the disk centrifuge to a polymer/hydrophobic liquidcomposition containing in excess of about 25% polymer solids. Theprimary effluent from the second centrifuge can be then blended with theprimary effluent from the first centrifuge to produce a finaldehydrated, concentrated polymer/emulsifying surfactant/hydrophobicliquid composition having in excess of about 60 weight % polymer solids,preferably in excess of about 70% polymer solids. The oil-rich raffinatefrom the second centrifuge can be recycled to improve the overallefficiency of the concentration process.

After removal of some portion of the hydrophobic liquid via one or morecentrifuge steps, a stabilizer is preferably added. In theory, anycompound which stabilizes the concentrated polymer and emulsifyingsurfactant-in-oil composition by reducing its tendency to compact orseparate can be used. Lauryl-methacrylate/methacrylic acid copolymershave been found to be especially effective non-aqueous polymer-in-oilsuspension stabilizers. For example, 99.5/0.5 w/wlauryl-methacrylate/methacrylic acid polymers having molecular weightsranging from about 10,000 to about 5,000,000 have been found to beuseful as stabilizers. An effective amount of the stabilizer should beused, preferably at least about 0.1 ppm, based on the weight of theconcentrated composition to be stabilized. Such stabilizers are preparedby conventional solution polymerization techniques (see, for example,U.S. Pat. No. 4,833,198).

FIG. 1 represents a flow diagram which is believed by the inventors torepresent a best mode embodiment of the instant invention. In thisfigure, starting emulsion polymer containing in excess of about 25weight % polymer solids, about 45% weight water, about 25% weight oil,and the balance inerts (i.e., the surfactant system, chelants, etc.) isstored in a Starting Emulsion Storage Tank 1. Effluent from tank flowsvia line 3 to evaporator 4. On route, the starting polymer emulsion isdiluted with suitable hydrophobic liquid from Diluent Tank 2 via line 5.The hydrophobic liquid is preferably the same hydrophobic liquid used inpreparation of the starting emulsion, recycled oil from the secondcentrifuge step, or some combination thereof. The dilution step resultsin a diluted water-in-oil polymer composition containing about 10% byweight polymer, about 18% by weight water, about 70% by weight oil, andthe balance inerts. An inline mixing device may be used to facilitatethorough mixing of the diluent and the starting polymer emulsion.

Evaporator 4 is a falling-film evaporator, preferably a thin filmevaporator, which is heated via hot oil or steam. Hot oil or steamenters the evaporator via line 6 and exits the evaporator via line 7.Evaporator 4, which preferably operates under vacuum, causes flashevaporation and removal of sufficient water to yield an evaporatorconcentrate which preferably contains less than about 3% by weightwater, more preferably from about 0.1 to about 1.5% water. Theevaporator concentrate exits the evaporator via line 8 generally at atemperature in excess of about 220° F. The contact time in theevaporator is generally about five (5) seconds. The evaporatorconcentrate preferably flows into an Evaporator Concentrate Storage Tank9. The evaporation step also generates a condenser feed stream 10 whichgenerally contains in excess of about 80% by weight water, between about0.5 and about 10% oil and less than about 1% inerts. The condensate feedstream 10 is cooled in heat exchanger 11 which preferably uses coolingwater via line 12 to remove heat from said condensate feed stream. Thecooled condensate is collected in Condensate Storage Tank 13. Theevaporator concentrate flows from Evaporator Concentrate Storage Tank 9through heat exchanger 14 into centrifuge 15. Preferably, a decantercentrifuge is used. Water is used as the cooling medium in heatexchanger 14 via lines 16. Thus, the evaporator concentrate, whichpreferably exits heat exchanger 14 at a temperature of less than about120° F. and which contains about 10-15% polymer solids, less than about3.0% water, between about 80 and about 90% oil and the balance inerts,flows to centrifuge 15. The centrifuge concentrates the evaporatorconcentrate by removing some portion of oil from the evaporatorconcentrate. A concentrated composition that is substantially a polymerand emulsifying surfactant in oil composition exits the centrifuge vialine 17. This effluent from centrifuge 15 preferably contains in excessof about 70 weight % polymer solids, less than about 2 weight % water,about 8-12 weight % oil and the balance inerts. The inerts comprise, toa large extent, the surfactant system of the original emulsion.Centrifuge 15 also produces a centrifuge raffinate stream which flowsvia line 18 to Raffinate Storage Tank 19. The raffinate from centrifuge15 generally contains less than about 5% polymer, and in excess of about90% oil. The raffinate is preferably fed via line 20 to centrifuge 21,which is preferably a partially ejecting disk centrifuge. Centrifuge 21produces a concentrate which generally contains in excess of about 25weight % polymer, less than about 1 weight % water, between about 50 and60 weight % oil and the balance inerts. This concentrate flows via line22, which then joins line 17, the centrifuge 15 concentrate line, toform a concentrated polymer-in-oil composition preferably containing inexcess of about 70% polymer solids, less than about 2% water, betweenabout 15 and 25% oil, and between about 5-10% inerts. This concentratedpolymer-in-oil composition flows via line 23 to Product Storage Tank 24.From Product Storage Tank 24, the concentrated polymer-in-oilcomposition flows through line 25, wherein alaurylmethacrylate/methacrylic acid stabilizer from Stabilizer StorageTank 26 is added vie line 27. Preferably, about 1.0 to 2.5% ofstabilizer (active basis) is added, based on the weight of thecentrifuge concentrate being treated. Additionally, from about 0.1 toabout 0.5%, based on concentrated polymer composition weight, ofinverting surfactant may be added from Storage Tank 28 via line 29 intoline 30 wherein the concentrated, stabilized composition flows throughstatic mixer 31 and into Finished Product Storage Tank 32 via line 33.

Partially ejecting disk centrifuge 21 also produces a raffinate streamwhich generally contains less than about 1 weight % polymer, less thanabout 0, weight % water, between about 95 and about 99.5 weight % oiland the balance inerts. This raffinate flows via line 34 to Oil StorageTank 35. From tank 35, the centrifuge 21 raffinate is either disposed ofor recycled into the process, for example to Diluent Tan 2, via pump 36.

The instant process is believed to be applicable to virtually anycomposition comprising a water soluble polymer in an aqueous phase whichis dispersed in a continuous hydrophobic liquid phase. Preferably,however, the water-in-oil polymer composition to be concentrated shouldnot contain an inverting surfactant, as inverting surfactants tend tointerfere with dehydration efficiency.

The instant invention is also directed to the compositions produced viathe instant process, to dehydrated emulsion polymer compositionsproduced via the use of a falling-film evaporator, particularly a thinfilm evaporator, and to emulsion polymer compositions resulting from theuse of one or more centrifuge steps to concentrate a dehydrated emulsionor suspension via separation of its hydrophobic liquid.

EXAMPLES

The following examples are intended to further demonstrate theinvention, but should not be interpreted as limiting the invention inany way.

Example I--Thin Film Evaporator

A starting polymer emulsion containing 25.45% of a 90/10 mole %acrylamide/AETAC polymer (commercially available from CalgonCorporation) was diluted using Kensol 61 to prepare a dilutedwater-in-oil emulsion containing the following components:

    ______________________________________                                                           Wt %                                                       ______________________________________                                        Polymer              10.42                                                    Water                18.12                                                    Oil                  69.80                                                    Emulsivying Surfactant System                                                                      1.66                                                     Total                100.00                                                   ______________________________________                                    

This diluted starting emulsion was charged to a feed tank using an airpowered diaphragm pump.

The evaporator rotor of a Votator 04-020 thin film evaporator wasstarted, the evaporator was brought up to operating temperature usingheated oil (120°-130° C.) and the unit's vacuum pump was started. Theoperating pressure of the evaporator was 13-15 torr.

The above described diluted emulsion was then fed to the evaporatoruntil steady state operation was established.

Based on operation of the thin film evaporator under steady stateconditions, the following mass balance was obtained, based on 100 lbs.of diluted feed.

    ______________________________________                                                Starting Evaporator Vapor                                                     Emulsion Concentrate                                                                              Discharge                                         ______________________________________                                        % Polymer 10.42      13.15      0.00                                          % Water   18.12      0.30       84.98                                         % Oil     69.81      84.42      14.17                                         % Inerts* 1.66       2.12       0.85                                                    100.00     79.20      20.80                                         ______________________________________                                         *Inerts include various emulsifying surfactants, sodium meta bisulfite,       citric acid and the sodium salt of ethylenediamine tetracetic acid.      

This example demonstrates the ability of a thin film evaporator todehydrate a starting polymer emulsion. The evaporator concentrateproduced in the above

example contained 0.3%, by weight, water, which means

that greater than 99.8% of the water in the starting emulsion wasremoved.

Example II--Decanter Centrifuge

The evaporator concentrate from Example I was fed to anAlfa-Laval/Sharples Model P-660 decanter centrifuge. This centrifugedeveloped centrifugal force of 3065× gravity.

The following material balance was obtained, based on 100 lbs. ofcentrifuge feed:

    ______________________________________                                                Example I                                                                     Centrifuge                                                                              Centrifuge Centrifuge                                               Feed      Concentrate                                                                              Raffinate                                        ______________________________________                                        % Polymer 13.15       78.50      2.59                                         % Water   0.3         1.81       0.06                                         % Oil     84.42       11.13      96.28                                        % Inerts  2.12        8.56       1.10                                                   100.00      13.92      86.08                                        ______________________________________                                    

The centrifuge step produced a polymer/emulsifyingsurfactant/hydrophobic liquid composition that contained 78.5% polymersolids and 1.81% water. This composition was a pourable polymer deliverysystem that is unique in the field of polymer science.

Example III--Stabilizer Addition

98.5% of centrifuge concentrate from Example II is fed through a Kinexstatic mixer with 1.5 gm 65% LMA/MAA copolymer. The discharge from themixer is a stable, pourable liquid composition that contained 77.3%polymer solids.

    ______________________________________                                                Example II                                                                    Centrifuge                                                                             65% LMA/MAA  Stabilized                                              Feed     Feed         Polymer                                         ______________________________________                                        % Polymer 78.50      --           77.32                                       % Water   1.81       --           1.78                                        % Oil     11.13      35           11.49                                       % Inerts  8.56       --           8.43                                        % LMA/MAA --         65           0.98                                        ______________________________________                                    

What is claimed is:
 1. A method for removing water from a water-in-oilpolymer composition which method comprises dehydrating said compositionin a falling-film evaporator, wherein said composition comprises: 1)about 25 to about 74.5%, based on total composition weight, of anaqueous phase comprising about 10 to about 60%, based on the weight of1), of a water soluble polymer and the balance water; 2) about 0.5 toabout 20%, based on total composition weight, of at least oneemulsifying surfactant; and 3) about 25 to about 74.5%, based on totalcomposition weight, of a hydrophobic liquid; wherein said polymer is awater soluble polymer prepared from one or more ethylenicallyunsaturated monomers; and wherein the molecular weight of said polymeris not substantially degraded in the dehydration step.
 2. The method ofclaim 1, wherein sufficient water is removed so as to produce apolymer/emulsifying surfactant/hydrophobic liquid composition whichcontains less than about 5% water.
 3. The method of claim 2, whereinsaid polymer/emulsifying surfactant/hydrophobic liquid compositioncontains less than about 3% water.
 4. The method of claim 3, whereinsaid polymer/emulsifying surfactant/hydrophobic liquid compositioncontains from about 0.1 to about 2.5% water.
 5. The method of claim 1,wherein said falling-film evaporator is a thin film evaporator.
 6. Themethod of claim 1, wherein said water-in-oil polymer composition has aninitial polymer concentration in excess of about 25% polymer, based ontotal composition weight, and wherein said composition is diluted with asuitable hydrophobic liquid so as to reduce the polymer concentration toless than 25% prior to passing said composition through saidfalling-film evaporator.
 7. The method of claim 6, wherein said polymerconcentration is reduced to between about 1 and about 15%.
 8. Apolymer/emulsifying surfactant/hydrophobic liquid composition producedby dehydrating a water-in-oil polymer composition in a falling-filmevaporator; wherein said water-in-oil oil composition comprises: 1)about 25 to about 74.5%, based on total composition weight, of anaqueous phase comprising about 10 to about 50%, based on the weight of1), of a water soluble polymer and the balance water; 2) about 0.5 toabout 20%, based on total composition weight, of at least oneemulsifying surfactant; and 3) about 25 to about 74.5%, based on totalcomposition weight, of a hydrophobic liquid; wherein said polymer is awater soluble polymer prepared from one or more ethylenicallyunsaturated monomers, and wherein the molecular weight of said polymeris not substantially degraded in said falling film evaporator.
 9. Thecomposition of claim 9, wherein said polymer/emulsifyingsurfactant/hydrophobic liquid composition contains less than about 3%water.
 10. The composition of claim 9, wherein said polymer/emulsifyingsurfactant/hydrophobic liquid composition contains from about 0.1 toabout 2.5% water.
 11. The method of claim 5, wherein said thin filmevaporator is arranged so that said water-in-oil polymer compositionenters said thin film evaporator and is distributed via the action offixed clearance rotor blades.
 12. The method of claim 11, wherein saidwater-in-oil polymer composition has an initial polymer concentration inexcess of about 25% polymer, based on total composition weight, andwherein said composition is diluted with a suitable hydrophobic liquidso as to reduce the polymer concentration to less than 25% prior topassing said composition through said thin film evaporator.
 13. Thecomposition of claim 8, wherein said falling film evaporator is a thinfilm evaporator.
 14. The composition of claim 13, wherein said thin filmevaporator is arranged so that said water-in-oil polymer compositionenters said thin film evaporator and is distributed via the action offixed clearance rotor blades.
 15. The method of claim 1, wherein saidfalling film evaporator operates under vacuum.
 16. The method of claim5, wherein said thin film evaporator operates under vacuum.